Chlorine dioxide, useful as a pulp mill bleaching agent, is produced chemically by reduction of an acid aqueous chlorate solution in accordance with the equation: EQU ClO.sub.3.sup.- +2H.sup.+ +e.sup.- .fwdarw.ClO.sub.2 +H.sub.2 O
where the electron e.sup.- is supplied by various reducing agents, for example, methanol, chloride ion and hydrogen peroxide. In many commercial processes for effecting this reaction, the acidity for the process is provided by sulfuric acid while the chlorate ions are provided by sodium chlorate. The presence of these species leads to the formation of some form of sodium sulfate as a by-product.
One particular embodiment of a commercial process is the so-called "R8" process of the assignee of this application, as described in U.S. Pat. No. 4,081,520, assigned to the assignee herein and the disclosure of which is incorporated herein by reference. Improvements in and modifications to that process also are described in the assignee's U.S. Pat. Nos. 4,465,658, 4,473,540 and 4,627,969, the disclosures of which are incorporated herein by reference.
In that chlorine dioxide generating process, the reaction medium is at a high total acid normality of sulfuric acid and is maintained at its boiling point under a subatmospheric pressure applied thereto. Methanol is used as a reducing agent for chlorate ions, resulting in the formation of chlorine dioxide in a substantially pure form. The boiling nature of the reaction medium produces steam which acts as a diluent for the gaseous chlorine dioxide, so as to prevent decomposition of the chlorine dioxide.
The sodium sulfate by-product builds up in the reaction medium after start-up until the solution is saturated with sodium sulfate, whereupon the sodium sulfate precipitates from the reaction medium. A slurry of the sodium sulfate is removed from the reaction vessel, the crystalline sodium sulfate is filtered therefrom and the mother liquor is recycled to the reaction zone after the addition of make-up quantities of sodium chlorate, sulfuric acid and methanol.
This process is highly efficient and rapidly produces chlorine dioxide in commercial quantities. As may be concluded from the above equation, for each mole of chlorine dioxide produced a mole of chlorate ion and hence of sodium ion is introduced to the reaction medium. The sodium ions combine with the sulfate ions introduced with the sulfuric acid, to produce a sodium sulfate, which may be sodium bisulfate or, more normally under the conditions of an R-8 process, the double salt sodium sesquisulfate, i.e., Na.sub.3 H(SO.sub.4).sub.2 (or NaHSO.sub.4.Na.sub.2 SO.sub.4), depending on the acidity of the solution.
Another sulfuric acid-based chlorine dioxide generating process, a low acidity "R3" process, as described in U.S. Pat. No. 3,864,456, the disclosure of which is incorporated herein by reference, produces neutral sodium sulfate as the by-product.
Such by-product sodium sulfate and sodium sesquisulfate (sometimes termed "saltcake"), generally have been employed to make up sulfur losses in the pulp mill.
However, the adoption of high substitution of chlorine by chlorine dioxide in the chlorination stage of the bleach plant has led to saltcake by-product production from the chlorine dioxide generating process exceeding the mill make-up requirements.
There exists a need, therefore, for a chlorine dioxide generating process which possesses the attributes of, for example, the R8 process, while, at the same time, producing less sodium sulfate by-product for the same level of production of chlorine dioxide. It is even more advantageous if, in addition to a lower saltcake production, caustic soda solution is co-produced together with ClO.sub.2, thus minimizing an NaOH/Cl.sub.2 imbalance presently existing in pulp mills.
It has previously been suggested in U.S. Pat. No. 4,129,484 to treat aqueous effluent from chlorine dioxide generating processes electrolytically to form an acid-enriched fraction from the original solution, which then may be recycled to the chlorine dioxide generator.
In order to utilize the by-product saltcake, it was proposed in the prior art to employ an electrochemical process to convert sodium sulfate into sulfuric acid and caustic soda solution in a three-compartment electrolytic cell, equipped with a cation-exchange membrane facing the cathode and an anionic membrane or a diaphragm facing the anode, wherein the saltcake solution is fed to the middle compartment. In an electric field, sodium and sulfate or hydrogen sulfate ions are transferred to the cathodic and anodic compartments respectively where they recombine with electrolytically-generated hydroxyl and hydrogen ions to form caustic soda and sulfuric acid, respectively.
Analogously, in a simplified process, a two-compartment electrolytic cell equipped with a cation exchange membrane was proposed to generate a mixture of sulfate and sulfuric acid in an anodic compartment along with caustic soda solution in the cathodic compartment.
The main drawback of these prior proposals was that the sulfuric acid solution produced had a low acid strength (less than 10 wt % H.sub.2 SO.sub.4), which imposes an excessive evaporative load on the chlorine dioxide generator, thereby rendering the process uneconomical and impractical.
Although higher sulfuric acid concentrations can be achieved in the electrochemical splitting of saltcake in the manner described in the prior art, the current efficiency for such a process is prohibitively low due to the leakage of H.sup.+ ions through the ion-exchange membrane. Such migration of hydrogen ions towards the cathode is related to a very high mobility of this ion relative to Na.sup.+ ions.
For example, in the aforementioned U.S. Pat. No. 4,129,484, current efficiencies as low as 9% for production of about 1 normal caustic soda solution and 39% for production of about 2M sulfuric acid were reported.